The synovial joint is the most common and most complex type of joint found in the body.

In this article we shall look at the anatomy of the structures of a synovial joint, how they work together and how they can go wrong.

Synovial Fluid

Synovial fluid has three primary functions:

Lubrication

Nutrient distribution

Shock absorption.

This fluid is found in the synovial cavity of a joint, which is the space enclosed by the articular capsule. Its nutritional abilities are vital for healthy cartilage, which has a very poor blood supply so relies on diffusion from the synovial fluid.

Articular Capsule

This is a fibrous capsule which is continuous with the periosteum of articulating bones. It consists of two layers:

Outer fibrous layer – made up white fibrous tissue, called the capsular ligament. This holds together articulating bones and supports the synovium.

Inner synovial layer (synovium) – a highly vascularised layer of connective tissue. It absorbs and secretes synovial fluid, and is responsible for the mediation of nutrient exchange between blood and joint.

Fig 1 – The structures of a typical synovial joint. Synovial fluid is found in the joint cavity

Articular Cartilage

The bones of a synovial joint are covered by a thin layer of hyaline cartilage which serves to line the epiphysis (end) of the bone.

The smooth surface it provides has two functions; it minimises friction upon joint movement and absorbs shock.

Accessory Ligaments

Accessory ligaments are separate ligaments or parts of the joint capsule. They are made up of bundles of dense regular connective tissue, which are highly adapted for resisting strain. This prevents any extreme movements that may damage the joint.

Bursae

A bursa is a small sac lined by synovial membrane and filled with synovial fluid. They are placed at key points of friction in a joint, providing the joint with free movement.

They can become inflamed following infection or irritation by over-use of the joint (bursitis). Examples of these friction points are where tendons run over the joint, as they do in the knee, a common location for bursitis.

Innervation

Joints have a rich nerve supply provided by articular nerves. Hilton’s Law states that the nerves supplying a joint also supply the muscles moving the joint and the skin covering their distal attachments.

The nerves of a joint transmit impulses which play a key role in proprioception – the ability of the body to tell where parts of it are.

Vasculature

Joints receive blood via articular arteries which arise from the vessels around the joint. The articular arteries are located within the joint capsule, mostly in the synovial membrane.

A common feature of the articular arterial supply is frequent anastomoses (communications) in order to ensure a blood supply to and across the joint regardless of its position. In practice this usually means arteries are above and below a joint, curving round each side of it and joining via small connecting vessels.

The articular veins accompany the articular arteries and are also found in the synovial membrane.

Clinical Relevance: Osteoarthritis

Osteoarthritis is the most common form of joint inflammation (arthritis). It stems from heavy use of articular joints over the course of many years, which can result in the wearing away of articular cartilage, and often the erosion of the underlying articulating surfaces of bones as well. The changes which occur are irreversible and degenerative. This results in the decreased effectiveness of articular cartilage as a shock absorber and lubricated surface, as well as the roughened edges causing further damage.

As a result of this degeneration, repeated friction can cause symptoms of joint pain, stiffness and discomfort. This condition usually affects joints that support full body weight, such as the hips and the knees.

Arthritis can also come about through other causes, including; (i) as a result of infection, due to the ease with which blood (and any associated bacteria) can enter the joint cavity via the synovial membrane; (ii) due to autoinflammatory causes, as in rheumatoid arthritis, or; (iii) as a result of infection but not involving infection of the joint itself, as in reactive arthritis.

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